Serotonin (5-hydroxy-tryptamine) (5-HT) receptors play an important role in many physiological and pathological functions like anxiety, sleep regulation, aggression, feeding and depression. The 5-HT receptors are distributed throughout the body and can be divided is into seven different 5-HT receptor subtypes, i.e. 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7, with different properties. The 5-HT6 receptor is mostly found in the central nervous system (CNS). From in situ hybridization studies it is known that the 5-HT6 receptor in rat brain is localized in areas like striatum, nucleus accumbens, olfactory tubercle and hippocampal formation (Ward et al., Neuroscience, 64, p 1105-1111, 1995).
Scientific research has revealed a potential therapeutic use for modulators of the 5-HT6 receptor, especially with regard to various CNS disorders. Blocking 5-HT6 receptor function has been shown to enhance cholinergic transmission (Bentley et al, Br J Pharmacol 126: 1537-1542, 1999; Riemer et al J Med Chem 46, 1273-1276). 5-HT6 antagonist have also been shown to reverse cognitive deficits in in vivo cognition models induced by the muscarinic antagonist scopolamine (Woolley et al. Phychopharmacolgy, 170, 358-367, 2003; Foley et al. Neuropsychopharmacology, 29 93-100, 2004). Studies have shown that 5-HT6 antagonists increase levels of glutamate and aspartate in the frontal cortex and dorsal hippocampus as well as acetylcholine in the frontal cortex. These neurochemicals are known to be involved in memory and cognition (Dawson et al., Neuropsychopharmacology, 25(5), p 662-668, 2001; Gerard et al., Brain Res., 746, p 207-219, 1997; and Riemer et al J Med Chem 46(7), p 1273-1276, 2003).
Acetylcholinesterase inhibitors increase the levels of acetylcholine in the CNS and are used in the treatment of cognitive disorders such as Alzheimer's disease. 5-HT6 antagonists may therefore also be used in the treatment of cognitive disorders (Bentley et al., Br. J. Pharmacol. (1999) 126, 1537-1542).
Studies have also shown that 5-HT6 antagonists increase the level of dopamine and noradrenaline in the medial prefrontal cortex (Lacroix et al. Synapse 51, 158-164, 2004). In addition, 5-HT6 receptor antagonists have been shown to improve performance in the attentional set shifting task (Hatcher et al. Psychopharmacology 181 (2):253-9, 2005). Therefore, 5-HT6 ligands are expected to be useful in the treatment of disorders where cognitive deficits are a feature, such as schizophrenia. Several antidepressants and atypical antipsychotics bind to the 5-HT6 receptor and this may be a factor in their profile of activities (Roth et al., J. Pharm. Exp. Therapeut., 268, 1402-1420, 1994; Sleight et al., Exp. Opin. Ther. Patents, 8, 1217-1224, 1998; Kohen et al., J. Neurochem., 66(1), p 47-56, 1996; Sleight et al. Brit. J. Pharmacol., 124, p 556-562, 1998; and Bourson et al., Brit. J. Pharmacol., 125, p 1562-1566, 1998).
Studies have described the potential use of 5-HT6 modulators in the treatment of epilepsy (Stean et al., Brit. J. Pharmacol. 127 Proc. Supplement 131P, 1999). 5-HT6 receptors have also been linked to generalized stress and anxiety states (Yoshioka et al., Life Sciences, 62, 17/18, p 1473-1477, 1998). 5-HT6 agonists have been shown to elevate levels of GABA in brain regions associated with anxiety and shown positive effects in models predictive of obsessive-compulsive disorder (Schechter et al. NeuroRx. 2005 October; 2(4): 590-611). The use of modulators for this receptor is therefore expected for a wide range of CNS disorders.
Moreover, a reduction in food intake in rats has been reported using 5-HT6 receptor modulators (Woolley et al. Neuropharmacology 41 (2001) 210-219; Chen et al., European J. Pharmacology 534 (2006) 77-82). 5-HT6 receptor modulators, such as 5-HT6 antagonists, may therefore also be useful in the treatment of feeding disorders like anorexia, obesity, bulimia and similar disorders, including but not limited to, type 2 diabetes.
Indole 5-HT6 antagonists are disclosed in WO 02/078693.